Communication circuit

ABSTRACT

In a communication circuit, an RFIC includes an IO terminal and a control IC includes an IO terminal. A variable capacitance element includes control terminals, a capacitance element with a capacitance value that is determined according to a control voltage, and a resistance voltage divider circuit configured to generate the control voltage by dividing a voltage inputted to the control terminals. One of the RFIC and the control IC supplies control data to the variable capacitance element via a signal line. The variable capacitance element, along with an antenna coil, constitutes an antenna circuit of an LC parallel resonance circuit, and sets a resonant frequency of the antenna circuit to be a predetermined frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication circuit preferably foruse in an RFID (Radio Frequency Identification) system or a near fieldcommunication (NFC: Near Field Communication) system, in whichcommunication with a target device is performed by an electromagneticfield signal.

2. Description of the Related Art

NFC is one of the near field communication standards using a frequencyband of 13 MHz, and expected to be applied to various terminalsincluding mobile communication terminals. A mobile communicationterminal using NFC typically has an RFIC for NFC built in a main body ofthe terminal, and the RFIC for NFC is connected to an antenna coil forNFC that is also built within the terminal main body. Further, theantenna coil is connected to a capacitance element so as to resonate ata communication frequency, and the capacitance element and the antennacoil constitute an antenna circuit. In addition, the antenna circuit andthe RFIC for NFC or the like constitute a wireless communication module(hereinafter referred to as “NFC module”).

While a communication frequency for the NFC module is previouslydetermined, a resonant frequency to which the antenna circuit is to betuned varies to some degree depending on its use conditions and aproduction tolerance. For example, circuit architecture of the antennacircuit as a resonance circuit is different between a reader/writer modeand a card mode. Accordingly, it is necessary to adjust the resonancecircuit according to the mode so that a predetermined resonant frequencyis maintained in both modes. Further, the use conditions changeaccording to an environment in which the NFC module is installed. Forexample, the resonant frequency of the antenna circuit changes dependingon whether or not there is metal near the NFC module.

If a frequency band of the antenna in the NFC module is sufficientlybroad, fine adjustment due to the difference in the use conditions isnot necessary. However, it has become difficult to ensure an adequateantenna size as recent terminals are increasingly downsized, and theantenna's bandwidth may not be broadened if the size of the antenna issmall. Therefore, it is necessary to adjust the resonant frequency toobtain an optimal value.

As one method of adjusting the resonant frequency, there is known anantenna circuit including a capacitor configured by a variablecapacitance element capable of changing a capacitance value by anapplied voltage (see, for example, Japanese Patent UnexaminedPublication No. 2009-290644). Alternatively, Japanese Patent UnexaminedPublication No. 2010-147743 discloses a circuit that switches betweenentire capacitance values by selectively connecting a plurality ofcapacitors.

FIG. 10 is an example of a communication circuit disclosed in JapanesePatent Unexamined Publication No. 2010-147743. In the drawing, anon-contact IC unit 47 is configured by a non-contact IC chip, anantenna parallel capacitor unit having a capacitor Cin, parallelcapacitors C1 to C3, and the switches SW1 to SW3, and an antenna L1.Values of electric capacitances of the capacitor Cin and the parallelcapacitors C1 to C3 are static. The switches SW1 to SW3 are circuits forswitching between ON and OFF of the parallel capacitors C1 to C3,respectively. After the non-contact IC unit 47 is incorporated in amobile telephone 1, a control IC 62 having a non-volatile memory builtin is connected to the non-contact IC unit 47. The control IC 62controls the switches SW1 to SW3 of the non-contact IC unit 47 to switchbetween ON and OFF of the switches SW1 to SW3.

However, when a variable-capacitance diode and a switching circuit areprovided, it is necessary to provide a space for mounting these activeelements, and there is often a case in which the resonant frequencychanges because distortion may easily occur since these elements areactive elements. Further, terminals for receiving and transmittingsignals and data for adjusting the resonant frequency of the antennacircuit, as well as lines for transmitting these signals and data, arerequired. In addition, a large number of capacitors and switches arenecessary in order to adjust the capacitance value in fine steps byswitching between the plurality of capacitors. This adverselycomplicates the circuit architecture, and increases the size of an IC.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a communicationcircuit capable of adjusting a capacitance value of an antenna circuitwithout increasing the number of terminals configured to receive andtransmit signals and data, the number of lines to transmit the signalsand data, and the number of capacitance elements, thus greatlysimplifying circuit architecture.

A communication circuit according to a preferred embodiment of thepresent invention includes an RFIC configured to perform modulation anddemodulation between a baseband signal and a high-frequency signal; acontrol IC configured to control the RFIC by receiving and transmittingdata including communication data; an antenna coil; and a variablecapacitance element configured to change a resonant frequency of anantenna circuit including the antenna coil.

An IO terminal of the RFIC and an IO terminal of the control IC areconnected by a signal line, and control terminals of the variablecapacitance element are connected to at least a portion of the signalline.

With this configuration, it is possible to adjust a capacitance value ofthe variable capacitance element without increasing the number ofterminals configured to receive and transmit signals and data, thenumber of lines to transmit the signals and data, and the number ofcapacitance elements, thus greatly simplifying the circuit architecture.

Preferably, the variable capacitance element includes a capacitanceelement with a capacitance value that is determined according to acontrol voltage, and a resistance voltage divider circuit configured togenerate the control voltage by dividing a voltage inputted to thecontrol terminals.

With this configuration, it is possible to adjust the capacitance valueof the antenna circuit without increasing the number of capacitanceelements, and thus to greatly simplify the circuit architecture.

Preferably, the resistance voltage divider circuit includes a pluralityof resistances each including a first terminal connected to each of thecontrol terminals; and a common line to which second terminals of theresistances are connected in common and through which the controlvoltage is outputted, and resistance values of the plurality ofresistances are determined to be in a ratio based on powers of 2 basedon a lowest value among the resistance values.

With this configuration, it is possible to achieve a linear relationshipbetween values of the control data and the control voltage for thevariable capacitance element with a relatively smaller number of lines(a number of bits) of data transmission lines, and to facilitate settingin multiple steps at constant resolution.

Preferably, the communication circuit further includes an external IOterminal connected to the signal line, and configured to receive andtransmit a signal from and to an external circuit.

With this configuration, it is possible to control the variablecapacitance element from the external circuit.

Preferably, the variable capacitance element and the RFIC aremonolithically configured as a single monolithic IC.

With this configuration, the number of the components is reduced, wiringof the data transmission lines is simplified to a large extent, and thusthe size and the weight of the communication circuit are significantlyreduced.

Preferably, the RFIC includes an operational mode switch configured toswitch an operational mode of the RFIC based on a signal outputted fromthe IO terminal of the control IC when the RFIC is turned on.

With this configuration, the IO port used by the control IC to controlthe RFIC and the output port for the control data to control thecapacitance value of the variable capacitance element preferably isconfigured as the same port, and it is therefore possible to efficientlyutilize a small number of IO ports.

Preferably, one of the RFIC and the control IC is configured todetermine a capacitance setting mode of the variable capacitance elementwhen power is turned on, and both of the RFIC and the control IC areconfigured to output control data for the variable capacitance elementto the IO terminal.

With this configuration, it is possible to supply the control data tothe variable capacitance element from either of the RFIC and the controlIC, and thus to provide a highly sophisticated communication circuit.

According to various preferred embodiments of the present invention, itis possible to provide a communication circuit capable of adjusting acapacitance value of an antenna circuit without increasing the number ofterminals configured to receive and transmit signals and data, thenumber of lines to transmit the signals and data, and the number ofcapacitance elements, thus greatly simplifying the circuit architecture.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a communication circuit 101 according toa first preferred embodiment of the present invention.

FIG. 2 is a detailed diagram of circuits provided between an RFIC 11 andan antenna coil 13.

FIG. 3 is a diagram illustrating a general configuration of a variablecapacitance element 14 along with circuits connected thereto.

FIG. 4 is an entire circuit diagram within the variable capacitanceelement 14.

FIG. 5 is a chart showing a relationship between 5-bit values from portsP21-P25 shown in FIG. 4 and a resistance voltage dividing ratio.

FIG. 6 is a circuit diagram of a communication circuit 102 according toa second preferred embodiment of the present invention.

FIG. 7 is a circuit diagram of a different communication circuit 103according to the second preferred embodiment of the present invention.

FIG. 8 is a flowchart of a processing of an RFIC provided for acommunication circuit according to a third preferred embodiment of thepresent invention.

FIG. 9A is a flowchart of a processing of the RFIC.

FIG. 9B is a flowchart of a processing of a control IC.

FIG. 10 is a circuit diagram of a communication circuit disclosed inJapanese Patent Unexamined Publication No. 2010-147743.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First PreferredEmbodiment

FIG. 1 is a circuit diagram of a communication circuit 101 according toa first preferred embodiment of the present invention. The communicationcircuit 101 is one example of an NFC module described above. Thecommunication circuit 101 preferably includes an RFIC 11, a control IC12, an antenna coil 13, and a variable capacitance element 14. The RFIC11 includes IO terminals 11P for GPIO (General Purpose Input/Output).Similarly, the control IC 12 includes IO terminals 12P for GPIO.

The RFIC 11 is programmed and configured to perform modulation anddemodulation between a baseband signal and a high-frequency signal. Thecontrol IC 12 may be called a host IC. The control IC controls the RFIC11, and receives and transmits data including communication data.

The variable capacitance element 14 includes control terminals 14P. Thevariable capacitance element 14 includes a capacitance element with acapacitance value that is determined according to a control voltage(bias voltage), and a resistance voltage divider circuit that generatesthe control voltage by dividing a voltage inputted to the controlterminals.

The variable capacitance element 14 and the antenna coil 13 of aparallel circuit are connected to two RX terminals (received signalterminals) of the RFIC 11.

The IO terminals 11P of the RFIC 11 and the IO terminals 12P of thecontrol IC 12 are connected by signal lines 15A, and the controlterminals 14P of the variable capacitance element 14 are connected to atleast a portion of the signal lines 15A and 15B. In the example shown inFIG. 1, the control terminals 14P are connected to all of the signallines 15A and 15B.

Further, the RFIC 11 and the control IC 12 are connected via datatransmission lines 16. For ports of the control IC 12 and the RFIC 11for the data transmission lines 16, a UART (universal asynchronousreceiver-transmitter circuit) preferably is provided, for example, anddata is transmitted and received via the data transmission lines 16based on a serial transfer method or a parallel transfer method.

As will be described later, the RFIC 11 and the control IC 12 receiveand transmit communication signals via the data transmission lines 16.The control IC 12 controls various setting or the like for the RFIC 11via the signal lines 15A. In addition, the RFIC 11 or the control IC 12supplies control data to the variable capacitance element 14 via thesignal lines 15A and 15B.

The variable capacitance element 14 and the antenna coil 13 constitutean antenna circuit which is an LC parallel resonance circuit, anddetermine a resonant frequency of the antenna circuit. The antenna coil13 is coupled with an antenna of a communication destination byelectromagnetic field coupling, and performs transmission and receptionfor short range communication.

FIG. 2 is a detailed diagram of circuit provided between the RFIC 11 andthe antenna coil 13. In FIG. 2, a circuit connected to two TX terminals(transmitted signal terminal) of the RFIC 11 is also shown. CapacitorsC21 and C22 in FIG. 2 are elements that are configured and arranged toadjust a degree of coupling between the RFIC 11 and the antenna coil 13.Further, the inductors L11 and L12 and the capacitors C11, C12, and C20constitute a transmission filter. For example, as the RFIC 11 operatespassively when the communication circuit operates in the card mode, theRFIC 11 generates a source voltage from an input signal inputted to theRX terminal and reads a reception signal, and performs load modulationof a circuit (load) connected to the TX terminal in transmission.Further, for example, since the RFIC 11 operates actively when thecommunication circuit operates in the reader/writer mode, the RFIC 11opens the RX terminal to transmit a transmission signal from the TXterminal in transmission, and opens the TX terminal to receive areception signal from the RX terminal in reception. In this manner, inthe communication circuit, impedance from the RFIC 11 toward the antennacoil 13 changes depending on the operation mode. As will be laterdescribed, the variable capacitance element 14 is controlled so that theresonant frequency of the antenna circuit is optimized depending on theoperation mode (so that the impedance from the RFIC 11 toward theantenna coil matches).

Here, the ESD protection elements 17A and 17B are connected between theground and both end terminals of the antenna coil 13, respectively. TheESD protection elements 17A and 17B let an electrostatic discharge surgefrom the antenna coil 13 out to an electrostatic discharge surge to theground to prevent an excess voltage from being applied to the RFIC 11.

FIG. 3 is a diagram illustrating a general configuration of the variablecapacitance element 14 along with circuits connected thereto. Referringto FIG. 3, the variable capacitance element 14 includes a controlvoltage application circuit 14R and a variable capacitance unit 14C. Acapacitance value between ports P11-P12 of the variable capacitance unit14C is determined according to a voltage applied between ports P13-P14.The control voltage application circuit 14R preferably is configured byresistance elements R21-R25 each including a first terminal connected toa GPIO port (GPIO0-GPIO4) of the RFIC 11 and a second terminal connectedin common. The lines in common connection are connected to the port P13of the variable capacitance unit 14C.

FIG. 4 is an entire circuit diagram within the variable capacitanceelement 14. The configuration of the control voltage application circuit14R is as shown in FIG. 3. The variable capacitance unit 14C ispreferably configured by capacitance elements C1-C6 and resistanceelements R11-R17. Ports P21-P25 of the control voltage applicationcircuit 14R are connected to the IO terminals 11P of the RFIC 11 asshown in FIG. 3. The RFIC 11 selectively sets the IO terminals 11P asthe GPIO ports to high level (source voltage) or low level (groundvoltage). Therefore, each of the resistance elements R21-R25 works as aresistance voltage divider circuit according to the level of thecorresponding IO terminal of the RFIC 11, and a control voltageaccording to its voltage dividing ratio and the source voltage isapplied to the port P13 of the variable capacitance unit 14C. As theport P14 of the variable capacitance unit 14C is grounded, the controlvoltage is applied between the ports P13-P14 of the variable capacitanceunit 14C. The effect of the voltage dividing will be described later indetail.

In the variable capacitance unit 14C, the control voltage is applied toboth end terminals of each of the capacitance elements C1-C6 via theresistance elements R11-R17. The resistance elements R11-R17 preferablyhave the same or substantially the same resistance value. The RFresistance elements R11-R17 apply the control voltage to the capacitanceelements C1-C6, and prevent an RF signal applied between the portsP11-P12 from leaking to the ports P13 and P14. Each of the capacitanceelements C1-C6 preferably is a ferroelectric capacitor configured suchthat a ferroelectric film is sandwiched between opposing electrodes. Asthe ferroelectric film changes its amount of polarization depending onan intensity of an electric field to be applied to change an apparentdielectric constant, it is possible to determine the capacitance valueby the control voltage.

FIG. 5 is a chart showing a relationship between 5-bit values from theports P21-P25 shown in FIG. 4 and the resistance voltage dividing ratio.The resistance values of the resistance elements R21-R25 shown in FIG. 4are determined to be in a ratio based on powers of 2 based on a lowestvalue among the resistance values. For example, the ratio between theresistance values of the resistance elements R21, R22, R23, R24, and R25is determined to be, for example, approximately 1:2:4:8:16. For example,when R21 is about 10 kΩ, R22 is about 20 kΩ and R25 is about 160 kΩ.

For example, when the port P21 is high level and all of the portsP22-P25 are low level, the resistance element R21 constitutes an upperarm of the resistance voltage divider circuit, and a parallel circuit ofthe resistance elements R22-R25 constitutes a lower arm. Alternatively,for example, when the ports P21 and P22 are high level and the portsP23, P24, and P25 are low level, a parallel circuit of the resistanceelements R21 and R22 constitutes the upper arm of the resistance voltagedivider circuit, and a parallel circuit of the resistance elementsR23-R25 constitutes the lower arm. In addition, as the resistance valuesof the resistance elements R21-R25 are determined to be in the ratiobased on powers of 2 based on a lowest value among these resistancevalues, the resistance voltage dividing ratio may take values in thefifth power of 2 (=32) ways depending on the combination of the portsP21-P25 in high level or low level.

The horizontal axis in FIG. 5 may also be referred to as 5-bit valuesfrom the ports P21-P25. Similarly, the vertical axis may also bereferred to as a voltage ratio to the source voltage.

Second Preferred Embodiment

FIG. 6 is a circuit diagram of a communication circuit 102 according toa second preferred embodiment of the present invention. Thecommunication circuit 102 is one example of the NFC module. Thecommunication circuit 102 includes an RFIC 111, a control IC 12, anantenna coil 13, and a variable capacitance element 114. In thisexample, the RFIC 111 and the variable capacitance element 114 areprovided within a single RFIC (variable-capacitance-element built-inRFIC) 110. The variable-capacitance-element built-in RFIC 110 preferablyis a monolithic IC in which a circuit unit of the RFIC 111 and a circuitunit of the variable capacitance element 114 are provided on an Sisubstrate through a series of semiconductor fabrication. With thisconfiguration, the number of the components is significantly reduced,wiring of the data transmission lines are greatly simplified to a largeextent, and thus the size and the weight of the communication circuitare greatly reduced.

Further, unlike the one described in the first preferred embodiment withreference to FIG. 1, the communication circuit 102 preferably isprovided with GPIO terminals 18. To the GPIO terminals 18, signal lines15A and 15B are connected. Therefore, it is possible to control thevariable capacitance element by an external circuit. The GPIO terminals18 are connected to, for example, a secure IC for RFID for receiving andtransmitting data to and from the control IC 12 or the RFIC 111.Further, a device such as an LED controlled by the control IC 12 isconnected.

FIG. 7 is a circuit diagram of a different communication circuit 103according to the second preferred embodiment. In this example, the RFIC111 and the variable capacitance element 114 are provided within thesingle variable-capacitance-element built-in RFIC 110, and a staticcapacitance element Cp is connected in series to the variablecapacitance element 114. The static capacitance element Cp is connectedoutside the variable-capacitance-element built-in RFIC 110.

A portion, instead of all, of the capacitance that is connected inparallel to the antenna coil 13 in this manner preferably defines thevariable capacitance element. Further, a portion of the capacitance thatis connected in parallel to the antenna coil 13 in this mannerpreferably is provided within the RFIC, and the remaining portionpreferably is connected outside the IC. It should be noted that thevariable capacitance element and the static capacitance element maypreferably be connected in parallel. By providing a combined capacitanceof a plurality of capacitance elements so as to be connected in parallelto the antenna coil 13, it is possible to significantly improve oroptimize a characteristic of the change of the resonant frequency of theantenna circuit (impedance from the RFIC) with respect to the change ofthe control voltage.

Third Preferred Embodiment

In a third preferred embodiment of the present invention, a processingof the RFIC (11 or 111) in the communication circuit described in one ofthe first preferred embodiment and the second preferred embodiment willbe described. FIG. 8 is a flowchart of the processing. Upon turning thepower of the RFIC on, the RFIC reads a state of GPIO0, which is one ofthe GPIO ports, and the processing branches according to its level. Ifthe GPIO0 is high level, the RFIC receives data to set a communicationprotocol from the control IC via a data transmission line (see the datatransmission lines 16 in FIG. 1), and sets the communication protocol(S1→S2). Then, the RFIC sets the GPIO port to be an “output port”, andoutputs the control data for the variable capacitance element via thesignal line (S3). With this, the capacitance value of the variablecapacitance element is set to a value corresponding to this controldata. For example, the capacitance value of the variable capacitanceelement is adjusted depending on whether the communication circuitoperates in the card mode or in the reader/writer mode. Then, the RFICturns the power of the high-frequency circuit unit on, performs a seriesof communications, and then turns the power of the high-frequencycircuit unit off (S4→S5→S6).

Further, if the state of the GPIO0 is low level, the RFIC receives datato write firmware from the control IC via the data transmission line,and writes the firmware of its own (the RFIC) (S1→S7).

As in this example, the IO ports that are used by the control IC tocontrol the RFIC and the output port for the control data to control thecapacitance value of the variable capacitance element are preferablyconfigured as the same port, and thus it is possible to efficientlyutilize a small number of IO ports.

Fourth Preferred Embodiment

In a fourth preferred embodiment of the present invention, the controlof the variable capacitance element in the communication circuitdescribed in one of the first preferred embodiment and the secondpreferred embodiment will be described.

FIG. 9A is a flowchart of the processing of the RFIC. Upon turning thepower of the RFIC on, the RFIC determines whether or not the control ofthe variable capacitance element is to be performed by itself (theRFIC). If the RFIC is set such that it (the RFIC) is to perform thecontrol, the RFIC sets the GPIO port to be the “output port”, andoutputs the control data for the variable capacitance element via thesignal line (S11→S12). Then, the RFIC turns the power of thehigh-frequency circuit unit on, performs a series of communications, andthen turns the power of the high-frequency circuit unit off(S13→S14→S15). If the RFIC is not set such that the control of thevariable capacitance element is to be performed by itself (the RFIC),the RFIC performs communication directly (S11→S13→S14→S15).

FIG. 9B is a flowchart of a processing of the control IC. Upon turningthe power of the control IC on, the control IC determines whether or notthe control of the variable capacitance element is to be performed byitself (the control IC). If the control IC is set such that it (thecontrol IC) is to perform the control, the control IC sets the GPIO portto be the “output port”, and outputs the control data for the variablecapacitance element via the signal line (S21→S22).

Which one of the RFIC and the control IC is to control the variablecapacitance element may be determined previously by the firmware of theRFIC or the control IC, or may be appropriately set according to themode. For example, the control IC may perform tuning (absorption ofproduction tolerance) of the capacitance value of the variablecapacitance element in manufacturing, and the RFIC preferablysignificantly improves or optimizes the capacitance value of thevariable capacitance element depending on the use conditions on the sideof an assembly manufacturer.

In this manner, either of the RFIC and the control IC preferablysupplies the control data to the variable capacitance element.

Other Preferred Embodiments

In the preferred embodiments described above, the example in which oneof the RFIC and the control IC supplies the control data to the variablecapacitance element has been described. However, both of the RFIC andthe control IC may supply the control data to the variable capacitanceelement. For example, it is possible to use a configuration in whichGPIO3 and GPIO4 of the GPIO ports of the control IC 12 shown in FIG. 1are turned to high impedance and GPIO0-GPIO2 specify lower 3 bits of the5-bit control data for the variable capacitance element 14, andGPIO0-GPIO2 of the GPIO ports of the RFIC 11 are turned to highimpedance and GPIO3 and GPIO4 specify upper 2 bit of the 5-bit controldata. With this, it is possible to realize control such that the RFIC 11roughly adjusts the capacitance value of the variable capacitanceelement 14, and then the control IC 12 finely adjusts the capacitancevalue of the variable capacitance element 14. The relationship betweenthe upper bits and the lower bits may be opposite.

Further, while the preferred embodiments described above preferably areconfigured such that the capacitance value of the variable capacitanceelement is set to be a certain value, it is possible to use aconfiguration in which the resonant frequency of the antenna circuit isswept, an error rate or the like of the communication data is detected,and the capacitance value of the variable capacitance element isautomatically optimized based on the detected error rate, for example.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. (canceled)
 2. A communication circuit comprising: an RFIC configured to perform modulation and demodulation between a baseband signal and a high-frequency signal; a control IC configured to control the RFIC by receiving and transmitting data including communication data; an antenna coil; and a variable capacitance element configured to change a resonant frequency of an antenna circuit including the antenna coil; wherein an IO terminal of the RFIC and an IO terminal of the control IC are connected by a signal line, and control terminals of the variable capacitance element are connected to at least a portion of the signal line.
 3. The communication circuit according to claim 2, wherein the variable capacitance element includes a capacitance element with a capacitance value that is determined according to a control voltage, and a resistance voltage divider circuit configured to generate the control voltage by dividing a voltage inputted to the control terminals.
 4. The communication circuit according to claim 3, wherein the resistance voltage divider circuit includes a plurality of resistances each including a first terminal connected to each of the control terminals, and a common line to which second terminals of the resistances are connected in common and through which the control voltage is outputted, and resistance values of the plurality of resistances are determined to be in a ratio based on powers of 2 based on a lowest value among the resistance values.
 5. The communication circuit according to claim 2, further comprising an external IO terminal connected to the signal line and configured to receive and transmit a signal from and to an external circuit.
 6. The communication circuit according to claim 2, wherein the variable capacitance element and the RFIC are configured as a single monolithic IC.
 7. The communication circuit according to claim 2, wherein the RFIC includes an operational mode switch configured to switch an operational mode of the RFIC based on a signal outputted from the IO terminal of the control IC when the RFIC is turned on.
 8. The communication circuit according to claim 2, wherein one of the RFIC and the control IC is configured to determine a capacitance setting mode of the variable capacitance element when power is turned on, and both of the RFIC and the control IC are configured to output control data for the variable capacitance element to the IO terminal.
 9. The communication circuit according to claim 2, wherein the RFIC and the control IC are connected to each other via data transmission lines.
 10. The communication circuit according to claim 2, wherein the antenna circuit is an LC parallel resonance circuit.
 11. The communication circuit according to claim 2, further comprising capacitors configured and arranged to adjust a degree of coupling between the RFIC and the antenna coil.
 12. The communication circuit according to claim 2, wherein an impedance from the RFIC to the antenna coil is configured to change based on an operation mode of the communication circuit.
 13. The communication circuit according to claim 2, further comprising electrostatic discharge elements connected between the antenna coil and ground.
 14. The communication circuit according to claim 2, wherein the variable capacitance element includes a control voltage application circuit and a variable capacitance unit.
 15. The communication circuit according to claim 14, wherein the control voltage application circuit includes resistance elements connected to the RFIC and in common.
 16. The communication circuit according to claim 14, wherein the variable capacitance unit includes capacitance elements including a ferroelectric film between a pair of electrodes, and resistance elements having the same or substantially the same resistance value.
 17. The communication circuit according to claim 6, wherein the single monolithic IC includes a Si substrate including a circuit unit of the RFIC and a circuit unit of the variable capacitance element provided thereon.
 18. The communication circuit according to claim 6, further comprising a static capacitance element connected in series to the variable capacitance element.
 19. The communication circuit according to claim 2, wherein one of the RFIC and the control IC is configured to output control data for the variable capacitance element to the IO terminal.
 20. A Radio Frequency Identification system comprising the communication circuit according to claim
 2. 21. A Near Field Communication system comprising the communication circuit according to claim
 2. 